1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly, to a zoom lens system, for use in a copying apparatus, where zooming is performed with a constant conjugate distance.
2. Description of the Prior Art
In a conventionally-used copying apparatus, in order to vary the magnification, the conjugate distance is varied by moving a mirror as a projecting lens moves. For such a copying apparatus, since the mirror is moved in order to vary the conjugate distance, a mechanism for moving the mirror is required. For this reason, the copying apparatus is disadvantageous in reducing size and cost.
In resent years, zoom lens systems have been proposed which are used for varying the magnification in a copying apparatus.
For example, Japanese laid-open Patent Application No. S59-67512 discloses an orthometa-type zoom lens system for use in a copying apparatus which lens system is arranged symmetrically with respect to an aperture stop and includes six lens elements grouped into four units. In the lens system, the most object side lens element and the most image side lens element are negative lenses. The half view angle thereof is 14.degree. to 18.degree..
U.S. Pat. No. 4,813,773 discloses a zoom lens system for use in a copying apparatus which lens system is arranged symmetrically with respect to an aperture stop and includes four lens elements grouped into four units. In the lens system, the most object side lens element and the most image side lens element are negative lenses. The half view angle thereof is 17.degree. to 21.degree..